I recall someone spotting cracks on their white iPhone the moment he took it out of the box at home. When he went back to the store they accused him of dropping it and wouldn't exchange it or something.
As for the color. I think dirt in the crack make it easiler to be seen if it is white. So it could be as well happening for black too.
I just noticed it today, with this thread. My white one has a barely noticeble crack on the bottom. Never dropped - damn, never even went out with it, I'm still waiting for the damn invisible shield to come.
I suspect it is NOT plastic. Various rumors around the 'net point to a ceramic and crystal coating material combination that is transparent to radio waves yet REALLY hard.
Pretty sure that they even said the back is plastic during the keynote about it.
iPhone Pro will be made of titanium